Klebsiella pneumoniae exhibiting a phenotypic hyper-splitting phenomenon including the formation of small colony variants

In this study, we characterized a Klebsiella pneumoniae strain in a patient with shrapnel hip injury, which resulted in multiple phenotypic changes, including the formation of a small colony variant (SCV) phenotype. Although already described since the 1960s, there is little knowledge about SCV phenotypes in Enterobacteriaceae. The formation of SCVs has been recognized as a bacterial strategy to evade host immune responses and compromise the efficacy of antimicrobial therapies, leading to persistent and recurrent courses of infections. In this case, 14 isolates with different resisto- and morpho-types were distinguished from the patient's urine and tissue samples. Whole genome sequencing revealed that all isolates were clonally identical belonging to the K. pneumoniae high-risk sequence type 147. Subculturing the SCV colonies consistently resulted in the reappearance of the initial SCV phenotype and three stable normal-sized phenotypes with distinct morphological characteristics. Additionally, an increase in resistance was observed over time in isolates that shared the same colony appearance. Our findings highlight the complexity of bacterial behavior by revealing a case of phenotypic "hyper-splitting" in a K. pneumoniae SCV and its potential clinical significance.

Case Report: Hip arthroplasty after fracture-related joint infection caused by extensively drug-resistant Klebsiella pneumoniae

This case-report focuses on a 23-year-old soldier suffering from a fracture-related hip joint infection (FRI) due to extensively drug-resistant Klebsiella pneumoniae and S. epidermidis. The patient underwent multiple septic revision surgeries including the removal of remaining shrapnel accompanied by last-resort antimicrobial therapy with cefiderocol and colistin. Additionally, the surgeries included repeated tissue sampling for microbiological and histopathological analysis. An antibiotic-loaded cemented filler containing cefiderocol was used to improve local antimicrobial therapy. The biopsies prior to and during hip replacement surgery confirmed successful microbe eradication. Hip arthroplasty restored hip joint function and significantly improved patient's quality of life. The utilization of a trabecular metal shell and a meta-diaphyseally anchored cementless hip stem ensured secure implant fixation and early patient mobilisation. An adjusted biofilm active oral antimicrobial therapy after arthroplasty intervention was continued to prevent early periprosthetic joint infection. This case emphasizes the difficulties of managing FRI and multidrug-resistant pathogens. It contributes valuable insight into navigating complex orthopedic cases while ensuring successful hip arthroplasty outcomes. In conclusion, early interdisciplinary collaboration, appropriate antimicrobial therapy along with tailored surgical interventions are crucial for managing such complex cases successfully.

Genomic analysis of the international high-risk clonal lineage Klebsiella pneumoniae sequence type 395

BACKGROUND

Klebsiella pneumoniae, which is frequently associated with hospital- and community-acquired infections, contains multidrug-resistant (MDR), hypervirulent (hv), non-MDR/non-hv as well as convergent representatives. It is known that mostly international high-risk clonal lineages including sequence types (ST) 11, 147, 258, and 307 drive their global spread. ST395, which was first reported in the context of a carbapenemase-associated outbreak in France in 2010, is a less well-characterized, yet emerging clonal lineage.

METHODS

We computationally analyzed a large collection of K. pneumoniae ST395 genomes (n = 297) both sequenced in this study and reported previously. By applying multiple bioinformatics tools, we investigated the core-genome phylogeny and evolution of ST395 as well as distribution of accessory genome elements associated with antibiotic resistance and virulence features.

RESULTS

Clustering of the core-SNP alignment revealed four major clades with eight smaller subclades. The subclades likely evolved through large chromosomal recombination, which involved different K. pneumoniae donors and affected, inter alia, capsule and lipopolysaccharide antigen biosynthesis regions. Most genomes contained acquired resistance genes to extended-spectrum cephalosporins, carbapenems, and other antibiotic classes carried by multiple plasmid types, and many were positive for hypervirulence markers, including the siderophore aerobactin. The detection of "hybrid" resistance and virulence plasmids suggests the occurrence of the convergent ST395 pathotype.

CONCLUSIONS

To the best of our knowledge, this is the first study that investigated a large international collection of K. pneumoniae ST395 genomes and elucidated phylogenetics and detailed genomic characteristics of this emerging high-risk clonal lineage.

Geno- and Phenotypic Characteristics of a Klebsiella pneumoniae ST20 Isolate with Unusual Colony Morphology

Klebsiella pneumoniae is a common member of the intestinal flora of vertebrates. In addition to opportunistic representatives, hypervirulent (hvKp) and antibiotic-resistant K. pneumoniae (ABR-Kp) occur. While ABR-Kp isolates often cause difficult-to-treat diseases due to limited therapeutic options, hvKp is a pathotype that can infect healthy individuals often leading to recurrent infection. Here, we investigated the clinical K. pneumoniae isolate PBIO3459 obtained from a blood sample, which showed an unusual colony morphology. By combining whole-genome and RNA sequencing with multiple in vitro and in vivo virulence-associated assays, we aimed to define the respective Klebsiella subtype and explore the unusual phenotypic appearance. We demonstrate that PBIO3459 belongs to sequence type (ST)20 and carries no acquired resistance genes, consistent with phenotypic susceptibility tests. In addition, the isolate showed low-level virulence, both at genetic and phenotypic levels. We thus suggest that PBIO3459 is an opportunistic (commensal) K. pneumoniae isolate. Genomic comparison of PBIO3459 with closely related ABR-Kp ST20 isolates revealed that they differed only in resistance genes. Finally, the unusual colony morphology was mainly associated with carbohydrate and amino acid transport and metabolism. In conclusion, our study reveals the characteristics of a Klebsiella sepsis isolate and suggests that opportunistic representatives likely acquire and accumulate antibiotic resistances that subsequently enable their emergence as ABR-Kp pathogens.

Antibiotic-Resistant Enterobacteriaceae in Wastewater of Abattoirs

Antibiotic-resistant Enterobacteriaceae are regularly detected in livestock. As pathogens, they cause difficult-to-treat infections and, as commensals, they may serve as a source of resistance genes for other bacteria. Slaughterhouses produce significant amounts of wastewater containing antimicrobial-resistant bacteria (AMRB), which are released into the environment. We analyzed the wastewater from seven slaughterhouses (pig and poultry) for extended-spectrum β-lactamase (ESBL)-carrying and colistin-resistant Enterobacteriaceae. AMRB were regularly detected in pig and poultry slaughterhouse wastewaters monitored here. All 25 ESBL-producing bacterial strains (19 E. coli and six K. pneumoniae) isolated from poultry slaughterhouses were multidrug-resistant. In pig slaughterhouses 64% (12 of 21 E. coli [57%] and all four detected K. pneumoniae [100%]) were multidrug-resistant. Regarding colistin, resistant Enterobacteriaceae were detected in 54% of poultry and 21% of pig water samples. Carbapenem resistance was not detected. Resistant bacteria were found directly during discharge of wastewaters from abattoirs into water bodies highlighting the role of slaughterhouses for environmental surface water contamination.

Extensively-drug-resistant Klebsiella pneumoniae ST307 outbreak strain from north-eastern Germany does not show increased tolerance to quaternary ammonium compounds and chlorhexidine

BACKGROUND

An outbreak of extensively-drug-resistant Klebsiella pneumoniae strain ST307 in a cluster of hospitals in north-east Germany gave rise to the assumption that the epidemiological success of the strain could be based on increased tolerance to biocides.

METHODS

The tolerance of the outbreak strain was compared with epidemiologically unrelated clinical isolates of K. pneumoniae, and reference strains of Pseudomonas aeruginosa (ATCC 15442) and Escherichia coli K12 (NCTC 10538). Tests were performed in a miniaturized assay based on European Standard EN 1040. The widely used biocides benzalkonium chloride (BAC) and didecyl dimethyl ammonium chloride (DDAC), their commercial formulation Descosept spezial (DS), and the antiseptic agent chlorhexidine digluconate (CHG) were selected as test substances. These biocides are used regularly in the hospitals involved in the outbreak.

FINDINGS

All biocides had a bactericidal effect against all tested strains in the quantitative suspension test within 5 min at typically used concentrations and dilutions. The effectiveness of BAC and DDAC alone and in combination, and CHG antisepsis were not impaired under tested conditions.

CONCLUSION

The outbreak strain did not show significantly increased tolerance towards biocides regarding the antiseptic. Thus, the epidemiological success of the strain has to be ascribed to other causes, such as inadequate hand hygiene of visitors.

Novel DiOC3 96-well real-time efflux assay for discovery of NorA efflux pump inhibitors in Staphylococcus aureus

The NorA efflux pump is one of the most studied efflux systems in Staphylococcus aureus and confers multidrug resistance to a variety of dyes and antimicrobial compounds. Hence, inhibition of the NorA efflux pump might be a viable option for restoring susceptibility to antibiotics like fluoroquinolones. Fluorescent real-time efflux assays are important tools to identify putative efflux pump inhibitors. Nevertheless, the number of available compounds for usage in Staphylococcus aureus is limited. Previously, a 3-dipropyloxacarbocyanine iodide (DiOC₃) efflux assay was published that circumvented problems associated with the usage of ethidium bromide, namely slow efflux and suggested mutagenicity. However, the DiOC₃ assay protocol was cuvette - based and therefore needs to be adapted to the 96-well plate format. Hence, we optimized this assay for usage with 96-well plates. The new assay allows for rapid high-throughput efflux pump inhibitor screening.

A Klebsiella pneumoniae ST307 outbreak clone from Germany demonstrates features of extensive drug resistance, hypermucoviscosity, and enhanced iron acquisition

BACKGROUND

Antibiotic-resistant Klebsiella pneumoniae are a major cause of hospital- and community-acquired infections, including sepsis, liver abscess, and pneumonia, driven mainly by the emergence of successful high-risk clonal lineages. The K. pneumoniae sequence type (ST) 307 lineage has appeared in several different parts of the world after first being described in Europe in 2008. From June to October 2019, we recorded an outbreak of an extensively drug-resistant ST307 lineage in four medical facilities in north-eastern Germany.

METHODS

Here, we investigated these isolates and those from subsequent cases in the same facilities. We performed whole-genome sequencing to study phylogenetics, microevolution, and plasmid transmission, as well as phenotypic experiments including growth curves, hypermucoviscosity, siderophore secretion, biofilm formation, desiccation resilience, serum survival, and heavy metal resistance for an in-depth characterization of this outbreak clone.

RESULTS

Phylogenetics suggest a homogenous phylogram with several sub-clades containing either isolates from only one patient or isolates originating from different patients, suggesting inter-patient transmission. We identified three large resistance plasmids, carrying either NDM-1, CTX-M-15, or OXA-48, which K. pneumoniae ST307 likely donated to other K. pneumoniae isolates of different STs and even other bacterial species (e.g., Enterobacter cloacae) within the clinical settings. Several chromosomally and plasmid-encoded, hypervirulence-associated virulence factors (e.g., yersiniabactin, metabolite transporter, aerobactin, and heavy metal resistance genes) were identified in addition. While growth, biofilm formation, desiccation resilience, serum survival, and heavy metal resistance were comparable to several control strains, results from siderophore secretion and hypermucoviscosity experiments revealed superiority of the ST307 clone, similar to an archetypical, hypervirulent K. pneumoniae strain (hvKP1).

CONCLUSIONS

The combination of extensive drug resistance and virulence, partly conferred through a "mosaic" plasmid carrying both antibiotic resistance and hypervirulence-associated features, demonstrates serious public health implications.

Extensively drug-resistant Klebsiella pneumoniae ST307 outbreak, north-eastern Germany, June to October 2019

From June to October 2019, 17 patients (six infected, 11 colonised) with an extensively drug-resistant (XDR) Klebsiella pneumoniae strain were notified from four Western Pomerania medical facilities. The XDR K. pneumoniae produced carbapenemases NDM-1 and OXA-48, and was only susceptible to chloramphenicol, tigecycline and cefiderocol. Synergistic activity was observed for the combination of aztreonam plus ceftazidime-avibactam. Genomic analyses showed all isolates belonged to K. pneumoniae sequence type 307. Control measures and further investigations are ongoing.

Automated and rapid identification of multidrug resistant Escherichia coli against the lead drugs of acylureidopenicillins, cephalosporins, and fluoroquinolones using specific Raman marker bands

A Raman-based, strain-independent, semi-automated method is presented that allows the rapid (<3 hours) determination of antibiotic susceptibility of bacterial pathogens isolated from clinical samples. Applying a priori knowledge about the mode of action of the respective antibiotic, we identified characteristic Raman marker bands in the spectrum and calculated batch-wise weighted sum scores from standardized Raman intensity differences between spectra of antibiotic exposed and nonexposed samples of the same strains. The lead substances for three relevant antibiotic classes (fluoroquinolone ciprofloxacin, third-generation cephalosporin cefotaxime, ureidopenicillin piperacillin) against multidrug-resistant Gram-negative bacteria (MRGN) revealed a high sensitivity and specificity for the susceptibility testing of two Escherichia coli laboratory strains and 12 clinical isolates. The method benefits from the parallel incubation of control and treated samples, which reduces the variance due to alterations in cultivation conditions and the standardization of differences between batches leading to long-term comparability of Raman measurements.

On-site blood culture incubation shortens the time to knowledge of positivity and microbiological results in septic patients

Introduction

To determine whether on-site incubation of blood cultures at the intensive care unit (ICU) improves not only the time to incubation but also time to positivity, time to knowledge of positivity and time to results (identification and antibiotic susceptibility testing).

Methods

This observational single-centre study in ICU patients with severe sepsis and septic shock investigated the impact of blood culture incubation immediately on-site at the ICU (ICU group) by comparison with traditional processing in a remote laboratory (LAB group) on different time intervals of blood culture diagnostics from obtaining blood to clinician notification of final result. The effect of on-site incubation was evaluated in Kaplan-Meier estimates for the time to positivity, time to knowledge of positivity and time to microbiological results and a linear mixed model was built.

Results

A total of 3,549 blood culture sets from 657 ICU patients were analysed: 2,381 in the LAB group and 1,168 in the ICU group. Overall, 660 (18.6%) blood culture sets were positive and 2,889 (81.4%) sets remained negative. On-site incubation was associated with reduced time to knowledge of positivity (46.9 h [CI 43.4–50.8 h] vs. 28.0 h [CI 23.6–32.2 h], p < 0.001) and reduced time to result (61.4 h [CI 58.4–64.8 h] vs. 42.1 h [CI 39.1–47.5 h], p < 0.001). In blood cultures processed instantaneously at the ICU compared to incubation in the remote laboratory within 4 h, the time to microbiological result was significantly reduced by 8.5 h (p < 0.001). Pre-existing anti-infective therapy had no significant impact on diagnostic time intervals.

Conclusions

Instantaneous incubation of blood cultures in the ICU compared to incubation in a remote laboratory significantly improves time to knowledge to positivity and time to result. These effects are even more pronounced during off-hours of the microbiological laboratory. The results underline the importance of 24/7 diagnostics to provide round-the-clock processing of blood culture samples in patients with sepsis and septic shock and an immediate to communication of the results to the clinicians.

Clinically Approved Drugs Inhibit the Staphylococcus aureus Multidrug NorA Efflux Pump and Reduce Biofilm Formation

Staphylococcus aureus has acquired resistance to antibiotics since their first use. The S. aureus protein NorA, an efflux pump belonging to the major facilitator superfamily (MFS), contributes to resistance to fluoroquinolones (e.g., ciprofloxacin), biocides, dyes, quaternary ammonium compounds, and antiseptics. Different compounds have been identified as potential efflux pump inhibitors (EPIs) of NorA that result in increased intracellular concentration of antibiotics, restoring their antibacterial activity and cell susceptibility. However, none of the currently known EPIs have been approved for clinical use, probably due to their toxicity profiles. In the present study, we screened approved drugs for possible efflux pump inhibition. By screening a compound library of approximately 1200 different drugs, we identified nilotinib, a tyrosine kinase inhibitor, as showing the best efflux pump inhibitory activity, with a fractional inhibitory concentration index of 0.1875, indicating synergism with ciprofloxacin, and a minimum effective concentration as low as 0.195 μM. Moreover, at 0.39 μM, nilotinib, in combination with 8 μg/mL of ciprofloxacin, led to a significant reduction in biofilm formation and preformed mature biofilms. This is the first description of an approved drug that can be used as an efflux pump inhibitor and to reduce biofilms formation at clinically achievable concentrations.

Impact of antibiotic administration on blood culture positivity at the beginning of sepsis: a prospective clinical cohort study

OBJECTIVES

Sepsis guidelines recommend obtaining blood cultures before starting anti-infective therapy in patients with sepsis. However, little is known of how antibiotic treatment before sampling affects bacterial growth. The aim of this study was to compare the results of blood cultures drawn before and during antibiotic therapy.

METHODS

Prospective clinical cohort study of septic patients. Adult intensive care unit patients with two or three blood culture sets at the beginning of sepsis between 2010 and 2017 were included. Patients with blood culture samples obtained before antibiotic therapy were compared with patients with samples taken during antibiotic therapy. Blood culture positivity, defined as presence of a microbiological pathogen, was compared between the groups. Logistic regression was performed to adjust the impact of different factors with respect to blood culture positivity.

RESULTS

In total, 559 patients with 1364 blood culture sets at the beginning of sepsis were analysed. Blood culture positivity was 50.6% (78/154) among patients with sepsis who did not receive antibiotics and only 27.7% (112/405) in those who were already receiving antibiotics (p <0.001). Logistic regression revealed antibiotic therapy as an independent factor for less pathogen identification (odds ratio 0.4; 95% CI 0.3-0.6). Gram-positive pathogens (28.3% (111/392) versus 11.9% (116/972); p <0.001) and also Gram-negative pathogens (16.3% (64/392) versus 9.3% (90/972); p <0.001) were more frequent in blood culture sets drawn before antibiotic therapy compared with sets obtained during antibiotic therapy.

CONCLUSIONS

Obtaining blood cultures during antibiotic therapy is associated with a significant loss of pathogen detection. This strongly emphasizes the current recommendation to obtain blood cultures before antibiotic administration in patients with sepsis.

Evaluation of loop-mediated isothermal amplification for the rapid identification of bacteria and resistance determinants in positive blood cultures

The use of molecular assays to rapidly identify pathogens and resistance genes directly from positive blood cultures (BCs) contribute to shortening the time required for the diagnosis of bloodstream infections. In this work, loop-mediated isothermal amplification (LAMP) assays have been examined for their potential use in BC diagnosis. Three different assays were applied. The commercially available eazyplex® MRSA test detects Staphylococcus aureus, S. epidermidis, mecA, and mecC. Two in-house assays [Gram-positive (GP) and Gram-negative (GN)] have been developed for the detection of streptococci, enterococci, vanA, vanB, Pseudomonas spp., Enterobacteriaceae, and the bla _CTX-M family. A total of 370 positive BCs were analyzed. LAMP test results were obtained within 30 min, including sample preparation. Amplification was measured by real-time fluorescence detection. The threshold time for fluorescence intensity values ranged from 6.25 to 13.75 min. The specificity and sensitivity of the assays varied depending on the target. Overall, from 87.7% of BCs, true-positive results were obtained, compared to routine standard diagnosis. Twenty-one tests were true-negative because of the lack of an appropriate target (5.7%). The concordance of positive test results for resistance genes with subsequent antibiotic susceptibility testing was 100%. From 15 BC bottles with mixed cultures, eazyplex® assays produced correct results in 73% of the cases. This study shows that LAMP assays are fast and cost-saving tools for rapid BC testing in order to expedite the diagnostic report and improve the antibiotic stewardship for sepsis patients.

Intestinal decolonization of Enterobacteriaceae producing extended-spectrum β-lactamases (ESBL): a retrospective observational study in patients at risk for infection and a brief review of the literature

Background

Multidrug-resistant Escherichia coli and other enteric bacteria producing extended-spectrum β-lactamases (ESBL) have emerged as an important cause of invasive infection. Targeting the primary (intestinal) niche by decolonization may be a valuable approach to decrease the risk of relapsing infections and to reduce transmission of ESBL-producing enteric pathogens.

Methods

In a retrospective observational study we evaluated the efficacy of intestinal decolonization treatment using orally administered colistin or other non-absorbable agents given for 2 to 4 weeks in adult patients with previous relapsing infection and persistent intestinal colonization with ESBL-positive Enterobacteriaceae (ESBL-E). Eradication success was defined as negative rectal swab or stool culture at the end of treatment and at follow up-2 weeks after treatment discontinuation.

Results

First-line decolonization treatment led to eradication of ESBL-E in 19/45 patients (42 %, 7/18 low-dose [4 × 1 million units] colistin, 3/12 high-dose [4 × 2 million units] colistin, 9/15 rifaximin [2 × 400 mg]), and secondary/salvage treatment was successful in 8/13 patients (62 %, 20 treatment episodes). Late follow-up showed that 7/13 patients (54 %) with successful initial or salvage decolonization became recolonized within 3 months after post-treatment assessment while all eight of the patients failing initial or salvage decolonization treatment with late follow-up remained colonized. A narrative review of the literature confirms the limited efficacy of non-absorbable antibiotics including conventional selective digestive tract decolonization (SDD)-like combination regimens for eradicating multidrug-resistant enteric bacteria from the intestinal tract.

Conclusions

At present, there is no clear evidence of a significant decolonization efficacy using single-drug treatment with oral non-absorbable antibiotics. More effective regimens are needed and a better definition of at risk patients is required for planning meaningful randomized controlled studies in this field.

Three Dimensional Checkerboard Synergy Analysis of Colistin, Meropenem, Tigecycline against Multidrug-Resistant Clinical Klebsiella pneumonia Isolates

The spread of carbapenem-non-susceptible Klebsiella pneumoniae strains bearing different resistance determinants is a rising problem worldwide. Especially infections with KPC (Klebsiella pneumoniae carbapenemase) - producers are associated with high mortality rates due to limited treatment options. Recent clinical studies of KPC-blood stream infections revealed that colistin-based combination therapy with a carbapenem and/or tigecycline was associated with significantly decreased mortality rates when compared to colistin monotherapy. However, it remains unclear if these observations can be transferred to K. pneumoniae harboring other mechanisms of carbapenem resistance. A three-dimensional synergy analysis was performed to evaluate the benefits of a triple combination with meropenem, tigecycline and colistin against 20 K. pneumoniae isolates harboring different β-lactamases. To examine the mechanism behind the clinically observed synergistic effect, efflux properties and outer membrane porin (Omp) genes (ompK35 and ompK36) were also analyzed. Synergism was found for colistin-based double combinations for strains exhibiting high minimal inhibition concentrations against all of the three antibiotics. Adding a third antibiotic did not result in further increased synergistic effect in these strains. Antagonism did not occur. These results support the idea that colistin-based double combinations might be sufficient and the most effective combination partner for colistin should be chosen according to its MIC.

Pimozide Inhibits the AcrAB-TolC Efflux Pump in Escherichia coli

Efflux pump inhibitors (EPIs) are attractive compounds to reverse multidrug-resistance in clinically relevant bacterial pathogens. In this study we tested the ability of the neuroleptic drug pimozide to inhibit the Escherichia coli AcrAB-TolC efflux pump, whose overproduction confers resistance to various antimicrobial agents. A real-time Nile red efflux assay in the AcrAB – overproducing strain 3-AG100 revealed that pimozide was capable of full inhibition of this pump at a concentration of 100 µM, which is far below its intrinsic MIC (>1mM). However, MIC assay demonstrated very little effect of pimozide with regard to reduction in MICs of various antimicrobial compounds.

Only oxacillin MICs were reduced twofold in the presence of pimozide at 100 and 200 µM.

Since pimozide did considerably enhance accumulation of ethidium bromide in a fluorescence assay, ethidium bromide MIC assays in the presence and absence of this putative EPI were performed. They revealed that pimozide was able to reduce the MICs of ethidium bromide by 4-fold. In line with previous reports we suggest that the capability of EPIs to restore the susceptibility to antimicrobial agents can be highly substrate-specific due to different substrate binding sites.

Determination of real-time efflux phenotypes in Escherichia coli AcrB binding pocket phenylalanine mutants using a 1,2'-dinaphthylamine efflux assay

To evaluate the importance of phenylalanine residues for substrate transport in the Escherichia coli efflux pump protein AcrB, we subjected Phe-to-Ala binding pocket mutants to a real-time efflux assay with the novel near-infrared lipophilic membrane probe 1,2'-dinaphthylamine (1,2'-DNA). All mutations, with the exception of F617A, led to considerable retardation of efflux. F610A was the point mutation with the most pronounced impact, followed by F628A, F615A, F136A, and F178A. This is the first study to demonstrate the importance of single phenylalanine residues within the AcrB binding pocket for real-time substrate transport.

Altered spectrum of multidrug resistance associated with a single point mutation in the Escherichia coli RND-type MDR efflux pump YhiV (MdtF)

OBJECTIVES

YhiV (MdtF) is an resistance nodulation division (RND) type efflux pump in Escherichia coli with significant homology to AcrB but usually expressed at a low level in clinical isolates. When overexpressed the pump confers decreased susceptibility to a variety of substances including erythromycin and ethidium bromide (EtBr). We characterized two mutants of E. coli E12 (DeltaacrB DeltaacrF) overexpressing yhiV that showed surprising differences in their spectrum of multidrug resistance (MDR).

METHODS

The two mutants obtained after repeated exposure of E. coli E12 to levofloxacin were tested for antimicrobial susceptibility to a variety of agents and for intracellular accumulation of selected pump substrates. Gene expression was studied by quantitative RT-PCR, and yhiV was sequenced. Gene inactivation and replacement were done by phage lambda-based homologous recombination.

RESULTS

Mutant DKO20/1 overexpressed yhiV, showed a wild-type yhiV sequence and had >2-fold increased MICs of fluoroquinolones, novobiocin, macrolides/ketolides, EtBr, oxacillin and Phe-Arg-beta-naphthylamide (PAbetaN, a putative efflux pump inhibitor) compared with the E12 parent. A second mutant, strain DKO1/17 that had the Val-610-->Phe point mutation in YhiV differed from DKO20/1 by faster growth, >2-fold increased MICs of linezolid and tetracycline, but >2-fold decreased MICs of PAbetaN, azithromycin and telithromycin. Inactivation of yhiV in DKO1/17 and reintroduction of the wild-type and mutant yhiV sequence confirmed that the differing MICs of most of the drugs were associated with the observed single point mutation. Intracellular drug accumulation studies with linezolid and PAbetaN were consistent with the MIC results.

CONCLUSIONS

The region around amino acid Val-610 in YhiV appears to be involved in determining recognition and efficiency of export of a number of MDR efflux pump substrates. This single point mutation in the periplasmic loop of the pump can increase resistance to a given drug such as a fluoroquinolone while decreasing resistance to another one.

Intracellular accumulation of linezolid in Escherichia coli, Citrobacter freundii and Enterobacter aerogenes: role of enhanced efflux pump activity and inactivation

OBJECTIVES

The oxazolidinone class of antibiotics such as linezolid have a narrow spectrum of activity that targets Gram-positive bacteria. We hypothesized that the poor activity of linezolid in Gram-negative bacteria is in part caused by relatively low intracellular concentration due to efflux.

METHODS

Using whole cell accumulation assays we estimated the intracellular concentration of linezolid in Escherichia coli and other Enterobacteriaceae. We included test strains with enhanced RND-type multidrug efflux pump activity and with genetic inactivation of the pump or functional inhibition by carbonyl cyanide m-chlorophenylhydrazone as inhibitor of the proton motive force or 1-(1-naphthylmethyl)-piperazine (NMP), an efflux pump inhibitor.

RESULTS

Consistent with susceptibility studies, enhanced pump activity caused decreased accumulation, and pump inactivation and inhibition caused increased accumulation, of linezolid. The accumulation levels in test strains of E. coli, Citrobacter freundii and Enterobacter aerogenes with functional pumps were lower than in control strains of Staphylococcus aureus and Enterococcus faecium, but were higher after pump inactivation and correlated with ethidium bromide and pyronin Y accumulation.

CONCLUSIONS

The intracellular concentration of linezolid is comparatively low owing to efficient efflux of the drug and could be increased substantially by inhibition of RND-type efflux pumps.

Multidrug efflux inhibition in Acinetobacter baumannii: comparison between 1-(1-naphthylmethyl)-piperazine and phenyl-arginine-beta-naphthylamide

OBJECTIVES

1-(1-Naphthylmethyl)-piperazine (NMP) has been shown to reverse multidrug resistance (MDR) in Escherichia coli overexpressing RND-type efflux pumps but there are no data on its activity in non-fermenters like Acinetobacter.

METHODS

Antimicrobial susceptibility in the absence and presence of NMP and, for comparison, phenyl-arginine-beta-naphthylamide (PAbetaN), another putative efflux pump inhibitor (EPI), was tested in laboratory and mutant strains with differing intracellular dye accumulation and expression of adeB, and in clinical isolates of Acinetobacter baumannii.

RESULTS

Based on a 4-fold or greater MIC reduction, the effects of both EPIs at low concentrations (25 mg/L) were limited. PAbetaN had a highly selective action on the reduction in the MIC of rifampicin and clarithromycin. At a higher concentration of the putative EPIs (100 mg/L), NMP was more active than PAbetaN. This effect was not limited to strains with adeB overexpression, but affected the susceptibility to linezolid, chloramphenicol and tetracycline most, and was enhanced in clinical isolates with reduced fluoroquinolone susceptibility.

CONCLUSION

NMP can partially reverse MDR in A. baumannii and differs substantially in its activity from that of PAbetaN.

Effect of 1-(1-naphthylmethyl)-piperazine, a novel putative efflux pump inhibitor, on antimicrobial drug susceptibility in clinical isolates of Enterobacteriaceae other than Escherichia coli

OBJECTIVES

1-(1-Naphthylmethyl)-piperazine (NMP) has been shown to reverse multidrug resistance (MDR) in Escherichia coli overexpressing RND type efflux pumps, but there is no data on its activity in Enterobacteriaceae other than E. coli.

METHODS

The antimicrobial susceptibilities of laboratory strains and 167 clinical isolates of Enterobacteriaceae to a variety of antimicrobial agents were determined in the absence and presence of NMP and, for comparison, of Phe-Arg-beta-naphthylamide (PAbetaN), another putative efflux pump inhibitor (EPI). A 4-fold or greater reduction of the MIC after EPI addition was considered significant.

RESULTS

NMP consistently reduced the MIC of linezolid in Citrobacter freundii, Enterobacter aerogenes and Klebsiella pneumoniae clinical isolates. Significant effects of NMP addition in >50% of tested isolates were also seen for levofloxacin, tetracycline and chloramphenicol in E. aerogenes, and for levofloxacin and tetracycline in K. pneumoniae, whereas no or minor effects were observed in Serratia marcescens. MDR reversal by NMP was more likely in isolates with decreased susceptibility to fluoroquinolones. In most fluoroquinolone-resistant strains the activity was sufficient to render isolates drug-susceptible at clinically achievable concentrations. The activity of PAbetaN was different from that of NMP, suggesting different modes of action of the two putative EPIs.

CONCLUSION

NMP has moderate activity in reversing MDR in many but not all members of the Enterobacteriaceae family including clinical isolates. Its effects on resistance reversal depend on bacterial species and drug, and are different from those seen with PAbetaN.

Effect of 1-(1-naphthylmethyl)-piperazine, a novel putative efflux pump inhibitor, on antimicrobial drug susceptibility in clinical isolates of Escherichia coli

OBJECTIVES

1-(1-Naphthylmethyl)-piperazine (NMP) has been shown to reverse multidrug resistance (MDR) in Escherichia coli overexpressing resistance-nodulation-cell division type efflux pumps, but there is no data on its activity in clinical isolates of E. coli.

METHODS

The antimicrobial susceptibility of 60 clinical isolates of E. coli to a variety of antimicrobial agents was determined in the absence and presence of NMP and, for comparison, of Phe-Arg-beta-naphthylamide (PAbetaN), another putative efflux pump inhibitor (EPI). The intracellular accumulation of ethidium bromide was measured to confirm efflux pump inhibition as the likely mechanism of action of NMP.

RESULTS

Based on a 4-fold or greater reduction of the MIC after the addition of NMP in >50% of the isolates, significant effects of NMP at a concentration of 100 mg/L were seen for levofloxacin, linezolid and ethidium bromide. The ethidium bromide MIC changes after NMP addition correlated with differences in the ethidium bromide intracellular accumulation as measured by fluorometry in whole cell accumulation experiments. The activity of PAbetaN was different from that of NMP, in particular regarding macrolide resistance reversal, suggesting different modes of action of the two putative EPIs.

CONCLUSIONS

NMP is moderately active in reversing MDR in clinical isolates of E. coli and can partially restore fluoroquinolone susceptibility through inhibition of efflux pumps.

Selected arylpiperazines are capable of reversing multidrug resistance in Escherichia coli overexpressing RND efflux pumps

Several arylpiperazines capable of reversing multidrug resistance (MDR) in Escherichia coli overexpressing acrAB and acrEF but not in pump-deficient mutant strains were identified. 1-(1-Naphthylmethyl)-piperazine, one of the more active compounds, enhanced susceptibility to fluoroquinolones and other agents and increased the intracellular concentration of levofloxacin and ethidium bromide, suggesting efflux pump inhibition as the mechanism of MDR reversal.